There exists a variety of methods to separate a mixture of components by chromatographic methods, the methods generally involving the flow of a mobile phase over a stationary phase. For example in liquid chromatography, the mobile phase is a liquid and typically the stationary phase is a solid. The components to be separated are located within the mobile phase and can either be liquids or solutions, the solutions comprising liquids and/or solids dissolved in a solvent. The stationary phase has the properties of an adsorbent upon which the components of the mobile phase can be adsorbed. In the chromatographic process, as the mobile phase flows along the stationary phase, the components continually adsorb and desorb onto the stationary phase at a rate specific for a particular component. Rate differences between the various components allow for their separation.
In conventional liquid chromatography, the stationary phase can comprise a packed column of particles where efficient packing of the particles is essential in allowing optimum separation of components. Preferably, the stationary phase has a high surface area afforded by the surface area of the particles. If the particles are porous, an inner surface area can also exist as defined by pore dimensions to increase the overall surface area. A continued challenge in the field of chromatography is to improve resolution of component separation. Poor resolution is shown where bands of different components overlap excessively and the resulting chromatographed solutions retain a mixture of components. For example, poorly packed columns can generate large gaps or channels by which the mobile phase can effectively bypass portions of the stationary phase, resulting in poor resolution. In contrast, relatively little overlap between the bands of components defines good resolution. A combination of pore dimension, particle size and uniformity of particle packing contribute to affect the resolution of component separation.
In certain instances, however, an extremely well-packed column having particles of a small diameter and/or pore dimension can reduce the flow rate of the mobile phase due to a restricted mobile phase pathway. Consequently, higher pressures are required to provide an acceptable chromatographic flow rate. And at any given pressure, longer elution times result as the particle size and/or pore dimensions decrease.
To overcome the problems inherent in packed-particle columns, stationary phases comprising a continuous network have been developed. This continuous network phase or monolith can comprise pores of an appreciable dimension and at the same time, eliminate gaps or channels that can arise from poorly packed columns. For example, U.S. Pat. No. 5,624,875 relates to methods for preparing inorganic porous materials having pores of various desired dimensions.
There remain challenges to design chromatography columns and in particular, capillary columns that provide optimum resolution without requiring the longer operation times and/or increased pressure conditions. Prior art capillary columns typically involve a stationary phase permanently positioned within a circular cross-section tube having inner diameters ranging from 5 .mu.m to 0.5 mm. The stationary phase comprises particles which are applied to an inner wall of the capillary at high pressures of up to a conventional maximum of 400 bar. Higher application pressures may be necessary depending on the length of a column. Example applications for such columns include capillary electrophoresis or high performance liquid chromatography.